AVALANCHE WARNING for January 6, 2010

AVALANCHE WARNING JANUARY 6, 2010

An Avalanche Warning is in effect for the mountains of West Central Montana. Heavy wet snowfall is accumulating on a very weak snowpack structure throughout our advisory area. This warning includes all aspects and elevations above 6000 feet in the Bitterroot, Rattlesnake and the Southern Swan and Mission Mountains near Seeley Lake.

Current Avalanche Danger

Above 6000 feet on all aspects, the avalanche danger is HIGH. All SNOTEL sites are reporting high Snow Water Equivalent (SWE) with the northern locations receiving the most precipitation. Stuart Peak and North Fork Jocko SNOTEL sites report close to 1.5 inches of SWE and most Bitterroot Range SNOTEL sites report .80 inches of SWE in less than 24 hours.

We have two major concerns with the current situation.

In areas that earlier reported surface hoar development last week, this new snow will easily overload that weak layer. On Monday, we received reports of small avalanches failing on this buried surface hoar layer in the Bitterroot Mountains near Lolo Pass and near Lost Horse west of Darby. The backcountry area near Lost Trail Ski Area also has a significant buried surface hoar layer.

The second concern is of course the layer of rotten nasty facets at the ground that are continuing to fail in stability tests as we have shown in our YouTube videos. This layer is widespread throughout Western Montana on all aspects above 6000 feet. The surface hoar is spotty. Slopes that avalanche on the surface hoar layer may easily entrain enough snow and weight to initiate failure of the faceted snow near the ground. These may be large destructive avalanches similar to what we experienced last year at this time.

Below 6000 feet, the avalanche danger is highly dependant on the amount of snow accumulating. Many of these areas had rain yesterday and are now refreezing so the avalanche danger is mostly LOW. Most of the recent snow accumulation is above 6000 feet.

Weather and Snowpack Analysis

We will be out Wednesday in the Rattlesnake and will have observers in all our normal locations Thursday so we will give a more thorough report Friday morning. I expect this avalanche warning will be short-lived as we are expecting the moisture to end within the next few hours and temperatures to drop quickly. Please review our recent YouTube videos here:

Recent snow stability testing.

Weather Forecast and Avalanche Outlook

The Missoula Office of the National Weather Service is forecasting the current precipitation event to end with a much cooler air mass to reside over the area with clearing skies and a ridge of high pressure approaching from the west. Expect temperatures to drop to near zero Wednesday night.

Avalanche conditions will strengthen as temperatures cool. It is going to take several weeks for the weakness at the base of the upper elevation snowpack to gain strength. I mentioned in an earlier advisory that if our area received an inch or more of SWE, I would expect to see widespread avalanche activity on all aspects especially in areas where the surface hoar was buried.

We rely heavily on the many excellent observations people are sending us this winter. They help us produce a more accurate avalanche forecast since our resources are limited.

If you get out and see avalanche activity or want to send us quick snow observations, send us a note at [email protected] or call us at 406-530-9766. 530-9SNO.